The monoclonal antibody 51.1 reacts with human CD1d, a member of the CD1 family with similarity to the non-polymorphic MHC Class I-like molecules. CD1d is a highly conserved single transmembrane receptor of the Immunoglobulin Superfamily. CD1d can associate with beta-microglobulin another feature showing similarity to MHC class I molecules, but can also exist as a nonglycosylated protein not in association with beta microglobulin. This suggests different control mechanisms for presenting glycolipid containing molecules to CD1d reactive NKT cells. Expression of CD1d is found on B cells of the periphery, in resting monocytes and cortical thymocytes. On intestinal epithelial cells (IEC) expression is polarized. Expression can also be found at low levels intracellularly in hepatocytes. In HCV (hepatitis C virus) livers, CD1d is highly expressed compared to normal controls.

The 51.1 monoclonal antibody has been shown to have functional activity; blocking the interaction of CD1d transfected cells with NKT cells.